Instrumento

As mentioned in an earlier section, ultrasonic transducers are used extensively in medical research. They are presently being used in ultrasonic imaging, where x-rays were used formerly. There are two great advantages to the use of ultrasonic images rather than x-rays. One is that no harmful radiation is produced that could have future harmful tissue-damaging effects on the body. Second, by varying the intensity of the transmitted ultrasonic wave, it is possible to obtain different depths of image penetration. In this manner it is possible to obtain images of organs that would normally be transparent to x-rays.

Figure 3-37 Object being scanned by an ultrasonic scanner.

Figure 3-38 Camera using a sonic system for automatic focusing.

(courtesy of Polaroid Co.)

A practical application of sonar was shown in Figure 3-29: a popular depth finder used by boaters and fishermen. The Polaroid Camera Company has developed a very successful ultrasonic ranging device that is used in many of their cameras for automatic focusing. In the camera illustrated in Figure 3-38, the ranging device can

measure the distance between the camera lens and the photographic subject to within approximately ±5% of the actual distance. A typical hospital installation of ultrasonic scanning is shown in Figure 3-39.

Review Questions

3-1. Explain how the dynamic microphone works. Why is it called a self-generating transducer?

3-2. To operate properly, why does an electret microphone require an onboard charge?

3-3. Why does the capacitor microphone have a high input impedance, whereas a dynamic microphone has a low input impedance?

3-4. What does triboelectricity mean? Why does it occur primarily with capacitor microphones?

3-5. Why have cartridges made with Rochelle salts been replaced with ceramic piezoelectric cartridges in microphones? Why is PZT a popular material for manufacturing piezoelectric sensors?

3-6. Explain how the ribbon microphone works.

3-7. In sonar imaging or small-object detection, why are higher-frequency waves used rather than lower-frequency waves?

3-8. Explain the functions of the six basic components that comprise an ultrasonic transducer.

3-9. What is acoustical impedance? How is it determined?

3-10. What is a major drawback in using sound for ranging purposes?

What are some advantages in using sound for this purpose?

Problems

3-11. Design a sonic ranging system, using block diagrams only, that can measure distances of several hundred feet and is temperature compensated.

3-12. Calculate the acoustical impedance for air. What would the matching impedance be for a sonic ranging device whose sonic emitter is made of iron?

3-13. Calculate the Doppler shift at a receiver site of a sonar signal in water whose transmitted frequency is 250 Hz and whose transmitter is travelling toward the receiver at a velocity of 6,7 m/s.

3-14. Explain how you would set up a device to measure the density changes in liquids and solids in a testing facility by using a sounding device and precise timing equipment.

References

Allocca, John A., and Allen Stuart

Transducers: Theory and Operation, Reston, VA. Reston, 1984

Biber, C., S. Ellin, E.

Shenk, and J.

Stempeck

The Polaroid Ultrasonic Ranging System. Audio Engineering Society, New York, NY, 1980.

Norton, Harry N., Sensor and Analyzer Handbook, Englewood Cliffs, NJ:

Prentice Hall, 1982

Chapter 4 - Magnetic Sensors

Chapter Objectives

1. To understand the theory and operation of the following magnetic field-sensing devices:

a. The magnetic reed switch b. The Hall effect transistor c. The inductive pickup coil d. The vibrating wire transducer

4-1 - Introduction

In this chapter we study those sensors that either use or interact with a magnetic field to cause the sensor to operate. This subject covers quite a wide range of sensing devices, so we will study only the more commonly used ones frequently encountered in industry.

The magnetic field referred to above is one that is usually produced by a permanent magnet. This magnet is often mounted externally to the transducer body and associated somehow with the measurand. A typical example is given below. We discuss the various configurations in which magnetic sensors are found; additionally, we will see how and why these configurations all work.

4-2 - Commonly Sensed Measurands

The measurands most often sensed using magnetic sensors are (1) position,

(2) motion, and (3) flow.

In all three cases the sensing is contactless, which is very desirable. However, as we will find out later in our discussions, there is a drawback to this kind of sensing produced by what is known as viscous damping. Viscous damping is an interaction between a magnetic field and a moving member reacting to that field and causing the member to have its motion impeded, so to speak, due to the attractive forces involved.

As we discuss the various types of magnetic sensor applications we will see how each of the three measurands-position, motion, and flow-is detected.

4-3 - Magnetic Reed Sensor

The magnetic reed sensor is perhaps the simplest of all magnetic sensors in its construction. It came into existence around the mid-1960s and has been manufactured in a variety of forms. The thing to bear in mind about this type of sensor is the fact that it is a digital device. In other words, it is either in one state or another (i.e., either electrically on or off, as in a relay contact). Its theory of operation and various physical configurations are discussed below.